The present invention relates to optimizing charging of a bootstrap capacitor wherein a bootstrap capacitor is charged by a circuit emulating a bootstrap diode.
A common half bridge gate driver circuit 100 driving a load is illustrated in FIG. 1. The gate driver circuit 100 includes a high side and a low side driver circuits DRV1 and DRV2 for driving high side and low side transistors 105a and 105b in a complementary fashion. In the illustrated circuit 100, it is necessary to provide voltage DC1 for the high side driver circuit DRV1, which is referenced to a different reference level than voltage DC2 provided for the low side driver circuit DRV2.
That is because the source of the high side transistor 105a is above the source of the low side transistor 105b. The high side driver circuit DRV1 is referenced to the source of the high side output transistor 105a. Thus the powering voltage to the high side driver circuit DRV1 must be above the powering voltage to the low side driver circuit DRV2.
To do this, a bootstrap circuit, illustrated in FIG. 2, has been employed including a bootstrap capacitor CBS and a diode DBS coupled to the voltage DC2. The diode DBS allows the bootstrap capacitor CBS to be charged to a high side floating supply voltage VBS above the source voltage at a switched node A while the low side transistor 105b is conducting and the high side transistor 105a is turned OFF. When the low side transistor 105b is turned OFF, the power supply voltage to the high side driver circuit DRV1 is approximately at a level of the voltage DC2 above the source voltage at the switched node A. That is because the capacitor CBS has been charged through the diode DBS from the supply voltage DC2. Accordingly, the high side floating supply voltage VBS for the high side driver has been increased above the level of DC2 which powers the low side driver circuit DRV2 using this bootstrap circuit.
In another circuit 300, shown in FIG. 3, the bootstrap diode DBS (FIG. 2) has been replaced by a bootstrap diode emulator circuit 302, used for charging the bootstrap capacitor CBS. The advantage of circuit 300 over circuit 101 (FIG. 2) is that the losses due to the diode are reduced.
FIG. 4 illustrates the bootstrap diode emulator circuit 302. Typically, such circuit uses an FET 405 having lower forward losses than a diode. The bootstrap diode emulator circuit 302 further employs a gate control circuit 410 for accepting a low side input signal LIN and driving the FET 405 and a dynamic back-gate biasing circuit 415. The dynamic back-gate biasing circuit 415 accepts the low side input signal LIN and is connected to the low side return node B (see FIG. 3) and the bootstrap capacitor CBS. The FET 405 is also connected to the bootstrap capacitor CBS and to the low side supply voltage DC2 VCC.
The gate control circuit 410 is shown in FIG. 5. It includes switches 520, 525, 530, 535, and 545; inverter circuits 505 and 515; and a source 510. The dynamic back-gate biasing circuit 415 is shown in FIG. 6. That circuit includes switches 620, 625, 630, and 635; two current sources 610 and 615; and an inverter 605. The circuit 700 of FIG. 7 illustrates the components of the gate control circuit 410 and the dynamic back-gate biasing circuit 415 combined with the rest of the circuit 300 of FIG. 3. The circuit 700 is the subject of U.S. patent application Ser. No. 10/712,893, which is incorporated herein by reference.
The circuit 700 possesses limitations, including conditions when the bootstrap capacitor cannot be fully charged because the low side input signal LIN to the low side driver is low even though the voltage VS at the switched node A is still low. In this condition, when the low side input signal LIN is low, because the bootstrap diode emulator is off, it cannot charge the bootstrap capacitor CBS. This deficiency may cause development of insufficient voltage across the bootstrap capacitor CBS to properly power the high side driver.